The present invention relates, in general, to the manufacture of a fluid-tight plastic container of the type formed from a sheet of plastic stock material carrying an interlocking closure strip which forms a fastener along the top edge, for opening and resealing the container. More particularly, the invention relates to a preheater assembly for softening the interlocking closure strip prior to severing and heat sealing the edges of bags formed from a supply of stock material.
The stock material from which such containers are formed is described in U.S. Pat. No. 3,988,184, issued Oct. 26, 1976. As therein disclosed, the stock material may be produced by folding a plastic sheet lengthwise. The folded sheet is continuously fed to a welding station where the opposed edges are joined to the two interlocking halves of a plastic closure strip. Each half of the strip is formed with a flange for use in securing the strip to the longitudinal edges of corresponding sides of the folded sheet. The fastener strip is fed from a supply reel and is aligned with the edges to which it is to be secured, and the aligned parts are then fed through a welding station where they are secured in a continuous sealing operation. The resulting stock material is then stored on a reel for later use in producing individual containers.
Another technique for manufacturing the stock material is to extrude the closure strip halves with the web, as the web is manufactured, thus avoiding the need to weld the closure strip in a separate process.
An alternative form of manufacturing the stock material is described in copending U.S. application Ser. No. 217,854 of Lester D. Siebert, entitled "Sealing Head For Plastic Film" and filed on June 12, 1988, which application is assigned to the assignee herein. In that copending application, the plastic sheet or web is fed into a welding station unfolded, and is aligned in the station with the two halves of the closure strip. As set forth in that application, two or more closure strips can be applied to the plastic web, depending upon the style and construction of the container to be formed. The halves of the closure strips are held in alignment with the plastic web and are fed through sealing stations, each of which has a sealing head which corresponds to one of the closure strip halves. The sealing heads heat the corresponding strips and the web as the aligned parts pass under the sealing head, so that a thin strip of the plastic is melted and the parts are welded together. Thereafter, the plastic web with the attached closure strip halves is folded and the interlocking closure strip halves are pressed together to provide an elongated folded plastic web secured, for example, at its longitudinal edges by a closure strip. This stock material is then wound on a storage reel for later formation of individual containers.
As described in U.S. Pat. No. 3,847,711, issued Nov. 12, 1974, the stock material formed in either of the two methods described above is formed into individual containers by means of a heated cutting assembly which includes a pressure/cutting bar arranged for reciprocal movement into and out of contact with the stock material. The material rests on a cooperating surface such as a sealing bar, or anvil, which also applies heat to the stock material. The material moves intermittently across the anvil, periodically stopping to allow the pressure/cutting bar to move down against against the anvil to sever the web of material and to weld the now-adjoining stock material edges. Thus, the cutting bar cuts laterally across the web of stock material to sever and edge seal individual containers formed from the material.
Although this process and the apparatus for carrying it out has worked well for many years, the speed of operation of the machinery for forming these containers is limited. This is due to the fact that in the region of the interlocking closure strip which forms the fastener, the stock material is much thicker than is the remainder of the web forming the body of the container. For example, the web may be less than 4 mils in thickness and when doubled over, less than 8 mils, whereas the two closure halves may produce a closure up to 50 mils thick. This difference in thickness imposes different heating requirements for different parts of the container stock material. As a result, a choice must be made during the manufacturing process between using a relatively low heat, where production is slowed to insure proper sealing of the fastener ends when a cut is made, without breaking the cutter, or a relatively high heat, which seals the interlocking fastener strips more quickly and securely so that production can be speeded up, but at the same time risks damage to the thinner portions of the web through overheating.